CN113395006A - Modular multilevel converter loss optimization control method under voltage unbalance - Google Patents

Modular multilevel converter loss optimization control method under voltage unbalance Download PDF

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CN113395006A
CN113395006A CN202110624509.3A CN202110624509A CN113395006A CN 113395006 A CN113395006 A CN 113395006A CN 202110624509 A CN202110624509 A CN 202110624509A CN 113395006 A CN113395006 A CN 113395006A
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loss
phase
bridge arm
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multilevel converter
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CN113395006B (en
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邓富金
王梦悦
赵纪峰
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Southeast University
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/53—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M7/537—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
    • H02M7/5387—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration
    • H02M7/53871—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/08—Circuits specially adapted for the generation of control voltages for semiconductor devices incorporated in static converters
    • H02M1/088—Circuits specially adapted for the generation of control voltages for semiconductor devices incorporated in static converters for the simultaneous control of series or parallel connected semiconductor devices
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/32—Means for protecting converters other than automatic disconnection

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Abstract

The invention discloses a loss optimization control method for a modular multilevel converter under voltage unbalance, which comprises the following steps: monitoring the current of each phase of upper bridge arm in real time, the capacitor voltage of each sub-module of each phase of upper bridge arm and the state of each sub-module switching device of each phase of upper bridge arm; calculating to obtain loss of each sub-module of each upper bridge arm of each phase by using bridge arm current, capacitor voltage and switch state; calculating to obtain the total loss of the upper bridge arm of each phase; calculating to obtain the average value of the total loss of the three-phase upper bridge arm; and calculating to obtain the difference value between the average value of the total loss of the three-phase upper bridge arms and the total loss of each phase upper bridge arm, and changing the carrier frequency of each phase upper bridge arm to realize the same total loss of each phase upper bridge arm so as to achieve the purpose of optimally controlling the loss of the modular multilevel converter. According to the invention, the loss optimization control of the modular multilevel converter is realized by changing the carrier frequency of each bridge arm, and compared with the conventional method, the loss optimization control method does not need to increase the construction cost of the modular multilevel converter, has small influence on the quality of output electric energy and has simpler control algorithm design.

Description

Modular multilevel converter loss optimization control method under voltage unbalance
Technical Field
The invention belongs to the field of multi-level power electronic converters, and particularly relates to a modular multi-level converter loss optimization control method under voltage unbalance.
Background
The Modular Multilevel Converter (MMC) is a Converter topology with bridge arms formed by cascading submodules, is different from the traditional two-level and three-level converters, adopts a mode that step waves approach sine waves, has the advantages of high waveform quality, strong fault handling capacity, low manufacturing difficulty and the like, and has wide application space in medium-voltage occasions such as new energy grid connection, motor driving and the like.
In actual operation, the situation that the voltage on the alternating current side of the modular multilevel converter is unbalanced is more, and when the modular multilevel converter is in an unbalanced operation state, the loss generated by a power switch and a diode in a bridge arm submodule among three phases is deviated, so that the service life of a switching element of a three-phase bridge arm of the modular multilevel converter is prolonged, the fault rate of the switching element is greatly different, and the normal and stable operation of the modular multilevel converter is influenced. Therefore, when the voltage on the alternating current side is unbalanced, the loss of a three-phase bridge arm is adjusted, and the three-phase loss deviation of the modular multilevel converter is further reduced, so that the method is important for stable operation of the modular multilevel converter system.
Aiming at the problem of loss optimization control of the modular multilevel converter, the loss of the modular multilevel converter is optimized by optimizing the topological structure of sub-modules, optimizing a sub-module capacitance voltage balance strategy or optimizing a modulation strategy of the modular multilevel converter in the conventional method, but the method has the defects of increasing the control difficulty of the modular multilevel converter, increasing the output voltage distortion rate of the modular multilevel converter and the like, and has certain difficulty in practical application.
Aiming at the problems, a loss optimization control method of the modular multilevel converter under the condition of voltage unbalance is designed.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to provide a loss optimization control method of a modular multilevel converter under the condition of unbalanced voltage.
The purpose of the invention can be realized by the following technical scheme:
a loss optimization control method for a modular multilevel converter under voltage unbalance comprises the following steps:
s1, monitoring the upper bridge arm current i of each phase in real timepa(t)，ipb(t)，ipc(t) monitoring the real-time capacitance voltage value U of the N sub-modules on each phase of bridge arm in real timeCn_j(j ═ a, b, c, N ═ 1,2, …, N), real-time monitoring of switching device state function S for each phasep_jn(j＝a,b,c，n＝1,2,…,N)；
S2, calculating to obtain the nth sub-module loss P of the j phase upper bridge arm by using the current, the voltage and the switching state of the bridge armloss_jn(j＝a,b,c，n＝1,2,…,N)；
S3, accumulating losses of sub-modules of each phase bridge arm to obtain total loss P of each phase bridge armloss_j(j＝a,b,c)；
S4, calculating to obtain the average value P of the total loss of the three-phase upper bridge armave；
S5, calculating total loss P of upper bridge arm of each phaseloss_j(j ═ a, b, c) and the average value P of total loss of three-phase upper bridge armaveDifference value Δ P ofloss_j(j＝a,b,c)；
S6, changing carrier frequency f of each phasecsa，fcsb，fcscTherefore, the total loss of the bridge arms on each phase is the same, and the optimal control of the loss of the bridge arms is realized.
Further, the j-phase nth sub-module switch device state function S in S1p_an，Sp_bn， Sp_cnExpressed as:
Figure BDA0003101625750000021
the sub-module investment is as follows: the first power switch T1 is in the on state, the second power switch T2 is in the off state, and the submodules are cut off as: the first power switch T1 is in an off state and the second power switch T2 is in an on state.
Further, the total loss P of the upper bridge arm sub-modules of the modular multilevel converter in S2loss_jnThe calculation method comprises the following steps:
Ploss_jn＝PconT1_jn+PconT2_jn+PconD1_jn+PconD2_jn+PswT1_jn+PswT2_jn+PswD1_jn+P swD2_jn ②
formula II, PconT1_jnFor the conduction loss, P, of the j-phase nth sub-module first power switch T1conT2_jnFor the conduction loss, P, of the j-phase nth sub-module second power switch T2conD1_jnThe conduction loss of the first diode D1 of the n-th sub-module of j phaseconD2_jnConduction loss of the second diode D2 of the n-th sub-module of j phaseswT1_jnFor the switching loss, P, of the j-phase nth submodule of the submodule first power switch T1swT2_jnFor the switching loss, P, of the j-phase nth submodule of the submodule second power switch T2swD1_jnFor the switching loss, P, of the j-phase nth submodule of the submodule first diode D1swD2_jnIs the switching loss of the j-phase nth submodule of the submodule second diode D2.
Further, PconT1_jn、PconT2_jn、PconD1_jn、PconD2_jnThe calculation method comprises the following steps:
Figure BDA0003101625750000031
formula III, Tf＝2π/ω，iT1_jnMagnitude of current, i, flowing through the first power switch T1 of the n sub-module for the j-phaseT2_jnMagnitude of current, i, flowing through the n sub-module second power switch T2 for the j-phaseD1_jnThe magnitude of the current, i, flowing through the first diode D1 of the n sub-modules for the j-phaseD2_jnMagnitude of current, V, flowing through the second diode D2 of the n sub-module for the j-phaseT0Is zero current on-state voltage drop, V, of the power switchD0Is zero current on-state voltage drop of the diode, RceIs the on-resistance of the power switch, RdIs twoThe on-resistance of the pole tube.
Further, PswT1_jn、PswT2_jn、PswD1_jn、PswD2_jnThe calculation method comprises the following steps:
Figure BDA0003101625750000041
in the formula IV, Eon() As a function of the conduction energy of the power switch, Eoff() As a function of the turn-off energy of the power switch, Erec() As a function of the reverse recovery energy of the diode, UCn_jAnd (t) is j-phase nth sub-module capacitor voltage.
Further, if bridge arm current ipj>0 and Sp_jnWhen i is equal to 1T1_jn＝0，iT2_jn＝0，iD1_jn＝ipj，iD2_jn0; if bridge arm current iau>0 and Sp_jnWhen i is equal to 0T1_jn＝0，iT2_jn＝ipj，iD1_jn＝0，iD2_jn0; if bridge arm current iau<0 and Sp_jnWhen i is equal to 1T1＝-ipj，iT2＝0，iD1＝0，iD20; if bridge arm current iau<0 and Sp_jnWhen i is equal to 0T1＝0，iT2＝0，iD1＝0，iD2＝-ipj。
Further, the total loss P of the upper bridge arm of the modular multilevel converter in S3loss_jThe calculation formula of (2) is as follows:
Figure BDA0003101625750000042
p in said S4aveThe calculation formula of (2) is as follows:
Figure BDA0003101625750000043
total loss of three-phase upper bridge arm in S5Difference value delta P between loss and average value of total loss of three-phase upper bridge armloss_jThe calculation formula of (2) is as follows:
ΔPloss_j＝Ploss_j-Pave⑦。
further, the S6 changes the carrier frequency f of each phasecsa，fcsb，fcscThe total loss of the upper bridge arms of each phase is the same, and the specific method comprises the following steps: if Δ Ploss_j>0, reducing the j phase carrier frequency; if Δ Ploss_j<0, then increase the j phase carrier frequency, finally make Δ Ploss_a＝ΔPloss_b＝ΔPloss_cI.e. Ploss_j(j＝a,b,c)＝Pave。
The invention has the beneficial effects that:
1. according to the loss optimization control method of the modular multilevel converter under the unbalanced voltage, the loss difference of the three-phase bridge arm when the voltage at the AC side is unbalanced is calculated, so that the carrier frequency of the three-phase bridge arm is adjusted, the loss optimization control of the modular multilevel converter under the unbalanced voltage at the AC side is realized, a capacitor voltage balance strategy or a modulation strategy of the modular multilevel converter is not required to be changed, the quality of the output voltage waveform of the modular multilevel converter is little, and the control logic design is simple and easy to implement;
2. according to the loss optimization control method of the modular multilevel converter under the unbalanced voltage, only the carrier frequency of a three-phase bridge arm needs to be changed, the topological structure of the submodule of the modular multilevel converter does not need to be changed, the construction cost of the modular multilevel converter is not increased, the method is easy to implement in the conventional modular multilevel converter system, and the method has strong practicability;
3. the loss optimization control method of the modular multilevel converter under the unbalanced voltage not only can adjust the loss balance of a three-phase bridge arm, but also can reduce the total loss of the modular multilevel converter to a certain extent and further reduce the operation cost of the modular multilevel converter.
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In order to more clearly illustrate the embodiments or technical solutions in the prior art of the present invention, the drawings used in the description of the embodiments or prior art will be briefly described below, and it is obvious for those skilled in the art that other drawings can be obtained based on these drawings without creative efforts.
Fig. 1 is a schematic diagram of a three-phase modular multilevel converter topology according to an embodiment of the invention;
FIG. 2 is a schematic diagram of a sub-module topology of an embodiment of the present invention;
FIG. 3 is a schematic flow chart of the overall method of an embodiment of the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The invention provides a loss optimization control method of a modular multilevel converter under the condition of voltage unbalance, aiming at the problem of loss optimization control of the modular multilevel converter, wherein topological structures of a three-phase MMC and submodules are shown in figures 1 and 2, the three-phase MMC is composed of six bridge arms, and each bridge arm comprises N (N is 1,2, …, N) Submodules (SM) with the same topological structure and a bridge arm inductor Larm(ii) a The submodule is in a half-bridge structure and consists of two diodes D1 and D2, two power switches T1 and T2 and a direct current capacitor C.
As shown in fig. 3, based on the three-phase MMC and the topology structure of the sub-modules, a loss optimization control method for a modular multilevel converter under voltage imbalance includes: monitoring the current of each phase of upper bridge arm in real time, the capacitor voltage of each sub-module of each phase of upper bridge arm and the state of each sub-module switching device of each phase of upper bridge arm; calculating to obtain loss of each sub-module of each upper bridge arm of each phase by using bridge arm current, capacitor voltage and switch state; calculating to obtain the total loss of the upper bridge arm of each phase; calculating to obtain the average value of the total loss of the three-phase upper bridge arm; and calculating to obtain the difference value between the average value of the total loss of the three-phase upper bridge arms and the total loss of each phase upper bridge arm, and changing the carrier frequency of each phase upper bridge arm to realize the same total loss of each phase upper bridge arm so as to achieve the purpose of optimally controlling the loss of the modular multilevel converter.
The method specifically comprises the following steps:
s1, monitoring the upper bridge arm current i of each phase in real timepa(t)，ipb(t)，ipc(t) monitoring the real-time capacitance voltage value U of the N sub-modules on each phase of bridge arm in real timeCn_j(j ═ a, b, c, N ═ 1,2, …, N), real-time monitoring of switching device state function S for each phasep_jn(j＝a,b,c，n＝1,2,…,N)；
S2, calculating to obtain the nth sub-module loss P of the j phase upper bridge arm by using the current, the voltage and the switching state of the bridge armloss_jn(j＝a,b,c，n＝1,2,…,N)；
S3, accumulating losses of sub-modules of each phase bridge arm to obtain total loss P of each phase bridge armloss_j(j＝a,b,c)；
S4, calculating to obtain the average value P of the total loss of the three-phase upper bridge armave；
S5, calculating total loss P of upper bridge arm of each phaseloss_j(j ═ a, b, c) and the average value P of total loss of three-phase upper bridge armaveDifference value Δ P ofloss_j(j＝a,b,c)；
S6, changing carrier frequency f of each phasecsa，fcsb，fcscTherefore, the total loss of the bridge arms on each phase is the same, and the optimal control of the loss of the bridge arms is realized.
The j-phase nth sub-module switching device state function S in S1p_an，Sp_bn，Sp_cnExpressed as:
Figure BDA0003101625750000071
the sub-module investment is as follows: the first power switch T1 is in an on state and the second power switch T2 is in an off state. The submodule is cut out as: the first power switch T1 is in an off state and the second power switch T2 is in an on state.
The total loss P of upper bridge arm sub-modules of the modular multilevel converter in S2loss_jnThe calculation method comprises the following steps:
Ploss_jn＝PconT1_jn+PconT2_jn+PconD1_jn+PconD2_jn+PswT1_jn+PswT2_jn+PswD1_jn+P swD2_jn ②
formula II, PconT1_jnFor the conduction loss, P, of the j-phase nth sub-module first power switch T1conT2_jnFor the conduction loss, P, of the j-phase nth sub-module second power switch T2conD1_jnThe conduction loss of the first diode D1 of the n-th sub-module of j phaseconD2_jnConduction loss of the second diode D2 of the n-th sub-module of j phaseswT1_jnFor the switching loss, P, of the j-phase nth submodule of the submodule first power switch T1swT2_jnFor the switching loss, P, of the j-phase nth submodule of the submodule second power switch T2swD1_jnFor the switching loss, P, of the j-phase nth submodule of the submodule first diode D1swD2_jnIs the switching loss of the j-phase nth submodule of the submodule second diode D2.
Wherein P isconT1_jn、PconT2_jn、PconD1_jn、PconD2_jnThe calculation method comprises the following steps:
Figure BDA0003101625750000081
formula III, Tf＝2π/ω，iT1_jnMagnitude of current, i, flowing through the first power switch T1 of the n sub-module for the j-phaseT2_jnMagnitude of current, i, flowing through the n sub-module second power switch T2 for the j-phaseD1_jnThe magnitude of the current, i, flowing through the first diode D1 of the n sub-modules for the j-phaseD2_jnMagnitude of current, V, flowing through the second diode D2 of the n sub-module for the j-phaseT0Is zero current on-state voltage drop, V, of the power switchD0Is zero current on-state voltage drop of the diode, RceIs the on-resistance of the power switch, RdIs a dipolarThe on-resistance of the tube.
PswT1_jn、PswT2_jn、PswD1_jn、PswD2_jnThe calculation method comprises the following steps:
Figure BDA0003101625750000082
in the formula IV, Eon() As a function of the conduction energy of the power switch, Eoff() As a function of the turn-off energy of the power switch, Erec() As a function of the reverse recovery energy of the diode, UCn_jAnd (t) is j-phase nth sub-module capacitor voltage.
If bridge arm current ipj>0 and Sp_jnWhen i is equal to 1T1_jn＝0，iT2_jn＝0，iD1_jn＝ipj，iD2_jn0; if bridge arm current iau>0 and Sp_jnWhen i is equal to 0T1_jn＝0，iT2_jn＝ipj，iD1_jn＝0，iD2_jn0; if bridge arm current iau<0 and Sp_jnWhen i is equal to 1T1＝-ipj，iT2＝0，iD1＝0，iD20; if bridge arm current iau<0 and Sp_jnWhen i is equal to 0T1＝0，iT2＝0，iD1＝0，iD2＝-ipj。
Total loss P of upper bridge arm of modular multilevel converter in S3loss_jThe calculation formula of (2) is as follows:
Figure BDA0003101625750000091
p in said S4aveThe calculation formula of (2) is as follows:
Figure BDA0003101625750000092
the difference value between the total loss of the three-phase upper bridge arm and the average value of the total loss of the three-phase upper bridge arm in the S5ΔPloss_jThe calculation formula of (2) is as follows:
ΔPloss_j＝Ploss_j-Pave⑦；
the S6 changes each phase carrier frequency fcsa，fcsb，fcscThe total loss of the upper bridge arms of each phase is the same, and the specific method comprises the following steps: if Δ Ploss_j>0, reducing the j phase carrier frequency; if Δ Ploss_j<0, then increase the j phase carrier frequency, finally make Δ Ploss_a＝ΔPloss_b＝ΔPloss_cI.e. Ploss_j(j＝a,b,c)＝Pave。
According to the invention, the loss difference of the three-phase bridge arm when the voltage at the alternating current side is unbalanced is calculated, so that the carrier frequency of the three-phase bridge arm is adjusted, and the loss optimization control of the modular multilevel converter is realized when the voltage at the alternating current side is unbalanced. Compared with the conventional method, the construction cost of the modular multilevel converter is not required to be increased, the influence on the quality of output electric energy is small, and the design of a control algorithm is simple. Meanwhile, the topological structure of the submodule of the modular multilevel converter is not required to be changed, the construction cost of the modular multilevel converter is not increased, the modular multilevel converter is easy to implement in the existing modular multilevel converter system, and the practicability is high.
In the description herein, references to the description of "one embodiment," "an example," "a specific example" or the like are intended to mean that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, the schematic representations of the terms used above do not necessarily refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.
The foregoing shows and describes the general principles, essential features, and advantages of the invention. It will be understood by those skilled in the art that the present invention is not limited to the embodiments described above, which are described in the specification and illustrated only to illustrate the principle of the present invention, but that various changes and modifications may be made therein without departing from the spirit and scope of the present invention, which fall within the scope of the invention as claimed.

Claims (8)

1. A loss optimization control method for a modular multilevel converter under voltage unbalance is characterized by comprising the following steps:
s1, monitoring the upper bridge arm current i of each phase in real timepa(t)，ipb(t)，ipc(t) monitoring real-time capacitance voltage values U of N sub-modules on each phase of bridge arm in real timeCn_j(j ═ a, b, c, N ═ 1,2, …, N), real-time monitoring of switching device state function S for each phasep_jn(j＝a,b,c，n＝1,2,…,N)；
S2, calculating to obtain the nth sub-module loss P of the j phase upper bridge arm by using the current, the voltage and the switching state of the bridge armloss_jn(j＝a,b,c，n＝1,2,…,N)；
S3, accumulating losses of sub-modules of each phase bridge arm to obtain total loss P of each phase bridge armloss_j(j＝a,b,c)；
S4, calculating to obtain the average value P of the total loss of the three-phase upper bridge armave；
S5, calculating total loss P of upper bridge arm of each phaseloss_j(j ═ a, b, c) and the average value P of total loss of three-phase upper bridge armaveDifference value Δ P ofloss_j(j＝a,b,c)；
S6, changing carrier frequency f of each phasecsa，fcsb，fcscTherefore, the total loss of the bridge arms on each phase is the same, and the optimal control of the loss of the bridge arms is realized.
2. The loss optimization control method for the modular multilevel converter under the voltage unbalance as claimed in claim 1, wherein the j-phase n-th sub-module switching device state function S1 isp_an，Sp_bn，Sp_cnExpressed as:
Figure FDA0003101625740000011
the sub-module investment is as follows: the first power switch T1 is in the on state, the second power switch T2 is in the off state, and the submodules are cut off as: the first power switch T1 is in an off state and the second power switch T2 is in an on state.
3. The loss optimization control method for the modular multilevel converter under the condition of voltage unbalance according to claim 2, wherein the total loss P of the upper bridge arm submodule of the modular multilevel converter in S2 isloss_jnThe calculation method comprises the following steps:
Ploss_jn＝PconT1_jn+PconT2_jn+PconD1_jn+PconD2_jn+PswT1_jn+PswT2_jn+PswD1_jn+PswD2_jn②
formula II, PconT1_jnFor the conduction loss, P, of the j-phase nth sub-module first power switch T1conT2_jnFor the conduction loss, P, of the j-phase nth sub-module second power switch T2conD1_jnThe conduction loss of the first diode D1 of the n-th sub-module of j phaseconD2_jnConduction loss of the second diode D2 of the n-th sub-module of j phaseswT1_jnFor the switching loss, P, of the j-phase nth submodule of the submodule first power switch T1swT2_jnFor the switching loss, P, of the j-phase nth submodule of the submodule second power switch T2swD1_jnFor the switching loss, P, of the j-phase nth submodule of the submodule first diode D1swD2_jnIs the switching loss of the j-phase nth submodule of the submodule second diode D2.
4. The loss optimization control method for the modular multilevel converter under voltage unbalance as claimed in claim 3, wherein P isconT1_jn、PconT2_jn、PconD1_jn、PconD2_jnThe calculation method comprises the following steps:
Figure FDA0003101625740000021
formula III, Tf＝2π/ω，iT1_jnMagnitude of current, i, flowing through the first power switch T1 of the n sub-module for the j-phaseT2_jnMagnitude of current, i, flowing through the n sub-module second power switch T2 for the j-phaseD1_jnThe magnitude of the current, i, flowing through the first diode D1 of the n sub-modules for the j-phaseD2_jnMagnitude of current, V, flowing through the second diode D2 of the n sub-module for the j-phaseT0Is zero current on-state voltage drop, V, of the power switchD0Is zero current on-state voltage drop of the diode, RceIs the on-resistance of the power switch, RdIs the on-resistance of the diode.
5. The loss optimization control method for the modular multilevel converter under voltage unbalance as claimed in claim 4, wherein P isswT1_jn、PswT2_jn、PswD1_jn、PswD2_jnThe calculation method comprises the following steps:
Figure FDA0003101625740000031
in the formula IV, Eon() As a function of the conduction energy of the power switch, Eoff() As a function of the turn-off energy of the power switch, Erec() As a function of the reverse recovery energy of the diode, UCn_jAnd (t) is j-phase nth sub-module capacitor voltage.
6. The loss optimization control method of the modular multilevel converter under the voltage unbalance as claimed in claim 5, wherein if the bridge arm current i ispj>0 and Sp_jnWhen i is equal to 1T1_jn＝0，iT2_jn＝0，iD1_jn＝ipj，iD2_jn0; if bridge arm current iau>0 and Sp_jnWhen i is equal to 0T1_jn＝0，iT2_jn＝ipj，iD1_jn＝0，iD2_jn0; if bridge arm current iau<0 and Sp_jnWhen i is equal to 1T1＝-ipj，iT2＝0，iD1＝0，iD2＝0；If bridge arm current iau<0 and Sp_jnWhen i is equal to 0T1＝0，iT2＝0，iD1＝0，iD2＝-ipj。
7. The loss optimization control method for the modular multilevel converter under the voltage unbalance as claimed in claim 6, wherein the total loss P of the upper bridge arm of the modular multilevel converter in S3 isloss_jThe calculation formula of (2) is as follows:
Figure FDA0003101625740000032
p in said S4aveThe calculation formula of (2) is as follows:
Figure FDA0003101625740000033
the difference value delta P between the total loss of the three-phase upper bridge arm and the average value of the total loss of the three-phase upper bridge arm in the S5loss_jThe calculation formula of (2) is as follows:
ΔPloss_j＝Ploss_j-Pave⑦。
8. the loss optimization control method for the modular multilevel converter under the voltage unbalance as claimed in claim 7, wherein the S6 changes each phase carrier frequency fcsa，fcsb，fcscThe total loss of the upper bridge arms of each phase is the same, and the specific method comprises the following steps: if Δ Ploss_j>0, reducing the j phase carrier frequency; if Δ Ploss_j<0, then increase the j phase carrier frequency, finally make Δ Ploss_a＝ΔPloss_b＝ΔPloss_cI.e. Ploss_j(j＝a,b,c)＝Pave。
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